1. Field of the Invention
The invention concerns a method for avoidance of image artifacts in a magnetic resonance system operating with pulse sequences, the magnetic resonance system having gradient coils for generation of fields gradient and a radio-frequency transmitter for emission of an RF excitation signal. The artifacts are being caused by interference fields directed transversely to the gradient fields.
2. Description of the Prior Art
Maxwell's equations establish that interference fields that are transverse to a magnetic gradient field inevitably occur upon the generation of the gradient field. The interference fields can be directly derived from Maxwell's equations and are often also designated as Maxwell terms. Given generation of an x-gradient in a magnetic resonance apparatus, a transverse z-gradient thus always also arises in the x-component of the magnetic field. These interference fields frequently generate image distortions and image artifacts.
A number of methods have been proposed for suppression of the effects of Maxwell term. For example, the additional phase arising in the image due to the interference fields can be mathematically removed, but this is not applicable when the spin phase itself represents a relevant measurement value, in particular given interferences of k-space trajectories. Examples of phase-sensitive sequences are the turbo spin echo (TSE) sequences and the SPI sequences (“echo planar imaging”).
DE 199 31 210 A1 discloses a further method, wherein it is proposed to compensate the phase position displacement that is caused by transversal magnetic interference fields by frequency shifting the excitation frequency and/or by using a gradient offset at the corresponding gradient coil. A continuous correction is proposed in order to keep the phase position displacement at zero. However, a specific, fixed measurement position is assumed, so that the method has proven unsuitable for multi-slice measurements, since ultimately suitable average values for the respective offset must be encountered that allow no optimal correction. A high current consumption is additionally generated due to the permanently-present gradient offset and the maximum performance of the gradient coils is reduced.